The invention generally relates to spectral enhancement systems for enhancing a spectrum of multi-frequency signals, and relates in particular to spectral enhancement systems that involve filtering and nonlinear operations. Conventional spectral enhancement systems typically involve filtering a complex multi-frequency signal to remove signals of undesired frequency bands, and then nonlinearly mapping the filtered signal in an effort to obtain a spectrally enhanced signal that is relatively background free.
In many systems, however, the background information may be difficult to filter out based on frequencies alone. For example, many multi-frequency signals may include background noise that is close to the frequencies of the desired information signal, and may amplify some background noise with the amplification of the desired information signal.
As shown in FIG. 1, a conventional spectral enhancement system may include one or more band pass filters 10, 12 and 14, each having a different pass band frequency and into each of which an input signal is presented as received at an input port 16. The system also includes one or more compression units 18, 20, 22 that provide different amounts of amplification. The outputs of the compression units 18-22 are combined at a combiner 24 to produce an output signal at an output port 26. If the frequencies of the desired signals (such as a vowel sound in an auditory signal) are either within a band pass frequency or are surrounded by substantial noise signals in the frequency spectrum, then such a filter and amplification system may not be sufficient in certain applications. Moreover, multi-channel compression by itself improves audibility but degrades spectral contrast. A weak tone at one frequency is strongly amplified so that it is concurrently audible with a strong tone at another frequency that is weakly amplified. The asymmetric amplification due to compression degrades the spectral contrast that was present in the uncompressed stimulus.
Increasing spectral contrast and simultaneously performing compression for the hearing impaired appears to yield a modest but significant improvement for speech perception in noise.
See, for example, “Spectral Contrast Enhancement of Speech in Noise for Listeners with Sensorineural Hearing Impairments: Effects on Intelligibility, Quality, and Response Times”, by T. Baer, B. C. J. Moore and S. Gatehouse, J. Rehabil. Res. Dev., vol. 30, no. 1, pp. 49-72 (1993). Certain other research demonstrates a strong benefit of using vowels with well-contrasted formants in the auditory nerves of acoustically traumatized cats and discusses its implications for hearing-aid designs. See, for example, “Frequency Shaped Amplification Changes the Neural Representation of Speech with Noise-Induced Hearing Loss,” by J. R. Schilling, R. L. Miller, M. B. Sachs and E. D. Young, Hear Res., vol. 117, pp. 57-70, March 1998; “Contrast Enhancement Improves the Representations of ε-like Vowels in the Hearing Impaired Auditory Nerve,” by R. L. Miller, B. M. Calhoun and E. D. Young, J. Acoustic Soc. Am., vol. 106, no. 2, pp. 157-68 (2002); and “Biological Basis of Hearing-Aid Design,” by M. B. Sachs, I. C. Bruce, R. L. Miller and E. D. Young, Ann Biomed. Eng., vol. 30, no. 2, pp. 157-168 (2002). An interesting analog architecture uses interacting channels to improve spectral contrast although without multi-channel syllabic compression. See, for example, “Spectral Feature Enhancement for People with Sensorineaural Hearing Impairments: Effects on Speech Intelligibility and Quality,” by M. A. Stone and C. B. J. Moore, J. Rehab. Res. Dev., vol. 29, no. 2, pp.39-56 (1992).
Digital systems have also been developed for providing detailed analysis of the input signal in an effort to amplify only the desired signal, but such systems remain too slow to fully operate in real time. For example, see Spectral Contrast Enhancement Algorithms and Comparisons,” by J. Yang, F. Lou and A. Nehoria, Speech Communications, vol. 39, January 2003. Moreover, such systems also have difficulty distinguishing between the desired signal and background noise.
There is a need therefore, for an improved spectral enhancement system that efficiently and economically provides an improved spectrally enhanced information signal.